Towards Immersed Boundary Methods for Complex Roughness Structures in Scale-Resolving Simulations

In many technical applications the effect of surface roughness on the local flow as well as on integral characteristics is significant. Understanding and modeling their effect is an ongoing challenge as there are plenty of surface structures caused by intention, manufacturing, or wear which have different or even contrary effects on the boundary layer flow. Scale-resolving simulations like direct numerical simulations are a valuable tool in this context as they provide highly-resolved view of the local effect of roughness on the flow. However, complex surface structures pose challenges to the generation of commonly used body-fitted structured computational grids. Immersed boundary methods (IBM) are a promising tool for bypassing this challenge. In this paper the IBM implemented in the CFD-solver OpenFOAM is qualified for scale-resolving simulations of turbulent channel flows over rough surfaces by introducing an additional mass-flow controller. By means of three characteristic test-cases the direct numerical simulations with IBM are verified against corresponding simulations with body-fitted grids. The excellent quantitative prediction of average flow quantities as well as turbulent statistics demonstrate the suitability of the method for the simulation of turbulent flows over arbitrary complex rough surfaces.